1. Field of the Invention
The present invention relates to a device and a method for phase synchronization and tracking in the case of transmissions.
The invention can be applied especially to the transmission of small-sized packets in satellite telecommunications, for wireless links in ground applications, for radiofrequency beam type communications etc.
The method according to the invention can be applied, for example, to all types of packet communications such as TDMA (Time Division Multiple Access), frequency hopping, bursts, etc, for a variety of radio applications (such as radiofrequency beams, satellite transmissions, short-range links in the millimeter bands etc).
It relates especially to small-sized terminals which therefore work at very low bit rates.
It can be applied to any constellation, for example of the M-QAM, M-PSK and APSK type.
2. Description of the Prior Art
The problem of synchronization of a modem in burst mode is always very difficult. The classic methods consist of the insertion of a “header” formed by symbols known to the receiver. In the symbols, a correlation is made that enables detection when a threshold is crossed. The symbols reduce the transmission efficiency because they do not contain payload information. It therefore proves to be desirable to limit their number to the maximum extent.
The use, for example, of turbocodes and recent developments in error-correction encoding, have amplified this problem. Indeed, these new codes enable a sharp reduction in the point of functioning of the error correction codes. This means that the synchronization of the modem must work in more difficult conditions (lower signal-to-noise ratio or SNR).
In satellite transmission as well as in RF transmission, the problem is especially crucial inasmuch as the propagation is good (in particular, there is neither any problem of multiple paths nor of fading). It is thus possible to hope to attain an Eb/N0 (the ratio between the energy transmitted per payload bit and the spectral noise density) type of point of functioning, equal to 2 or 3 dB, giving a C/N0 (BPSK carrier/noise) ratio in the region of 0 dB.
To resolve this problem, an estimation is made of the maximum likelihood of the reference sequence sought.
Another approach consists in using the protection provided by the error correction codes to make the method more robust by benefiting from the processing gain resulting from the error correction code. This is the approach chosen in the thesis by C. Morlet [1] Démodulateur embarqué multiporteur pour service multimédia par satellite ENST (“Multicarrier Integrated Demodulator for Multimedia Services by Satellite”, ENST, reference 000E27 obtained in September 2000” and entitled Tentative Decision. This approach however has three major drawbacks:                The synchronization depends on the code used and therefore links these two parts of the modem which are generally independent,        The method is not very robust with respect to frequency errors,        While it is well-adapted in the use of classic convolution waveforms, the method is more complex and difficult to use with waveforms using turbocodes. Since turbocodes work by iteration, several iterations have to be performed to obtain a sufficiently low error rate at output to enable synchronization in the feedback loop. In practice, this implies a major computation cost (in fact, the operation is limited to half an iteration or to one iteration) and also a substantial processing time, which may be crippling in terms of latency.        
Satellite burst transmissions are also widely used for small stations which, owing to the power and antenna size used, have access to only low bit rates in terms of link budget. This makes them more sensitive to frequency errors. Indeed, the higher the carrier frequency, the greater the error generated by the VCO (Voltage Control Oscillator) used. For example, for a carrier frequency of 10 GHz, a bit rate of 64 kBits/s, a value ΔF*Ts equal to 10−3, where ΔF corresponds to the frequency error (the difference between the frequency of the signal and the theoretical frequency) and Ts is the duration of a symbol, the precision is equal to dt/t=10−8. This is far too much for a driver whose price and size are compatible with terminals used by the general public. The solution in which a high stability driver is used is not acceptable from an economic point of view in terminals of this type.
The U.S. Pat. No. 5,170,415 describes a method in which the learning sequence at the beginning of a burst is used firstly to acquire the time synchronization and secondly to obtain the phase synchronization.
The US patent No 2001/0017897 discloses a method that uses a phase weighting method that is inversely proportional to the amplitude to accelerate the convergence of the phase loop in the presence of phase jitter.
The object of the present invention relates especially to a method and device that use especially one or more assumptions relating to frequency deviation and a piece of information on the quality of the symbols in the phase loop.